1. Technical Field of the Invention
The present invention relates to a pattern forming method utilizing a resist composition used in the process of producing a semiconductor such as IC or producing a circuit board of liquid crystal, thermal head or the like or in the lithography step of other photo-application processes. More specifically, the present invention relates to a pattern forming method utilizing a resist composition suitable for exposure by an immersion-type projection exposure apparatus using a light source of emitting far ultraviolet light at a wavelength of 300 nm or less.
2. Description of the Related Art
Along with the progress in the development of refined semiconductor devices, the trend is moving into shorter wavelength of exposure light source and higher numerical aperture (high NA) of projection lens. At present, exposure machines with NA of 0.84 have been developed, where an ArF excimer laser having a wavelength of 193 nm is used as the light source. As commonly well known, these can be expressed by the following formulae:(Resolving power)=k1·(λ/NA)(Focal depth)=±k2·λ/AN2 wherein λ is the wavelength of exposure light source, NA is the numerical aperture of projection lens, and k1 and k2 are constants related to the process.
In order to realize still shorter wavelength and higher resolving power, studies are being made on exposure machines where an F2 excimer laser having a wavelength of 157 nm is used as the light source. However, the lens material used for the exposure apparatus so as to realize shorter wavelength and the material used for the resist are very limited and therefore, it is extremely difficult to stabilize the production cost or quality of the apparatus and materials. This may lead to a failure in procuring the exposure apparatus and resist having sufficiently high performance and stability within a required time period.
With respect to the technique of elevating the resolving power in optical microscopes, a so-called immersion method of filling a high refractive-index liquid (hereinafter sometimes referred to as an “immersion liquid”) between the projection lens and the sample has been conventionally known.
As for the “effect of immersion”, assuming that the wavelength of exposure light in air is λ0, the refractive index of immersion liquid to air is n, the convergence half-angle is θ and NA0=sin θ, the above-described resolving power and focal depth when immersed can be expressed by the following formulae:(Resolving power)=k1·(λ0/n)/NA0 (Focal depth)=±k2·(λ0/n)/NA02 
That is, the effect of immersion is equal to use of an exposure wavelength of 1/n. In other words, in the case of a projection optical system with the same NA, the focal depth can be made n times larger by the immersion.
This is effective for all pattern profiles and can be combined with super-resolution techniques such as phase-shift method and modified illumination method which are being studied at present.
Examples of the apparatus where this effect is applied to the transfer of a fine image pattern of semiconductor device include those described in JP-A-57-153433 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-7-220990, JP-B-63-49893 (the term “JP-B” as used herein means an “examined Japanese patent publication”) and JP-A-6-124873, but these patent publications are not discussing on a resist suitable for immersion exposure techniques.
JP-A-10-303114 states that the control of refractive index of the immersion liquid is important, because change in the refractive index of immersion liquid causes deterioration of the projected image due to wavefront aberration of the exposure machine. This patent publication discloses a technique of controlling the temperature coefficient of refractive index of the immersion liquid to fall within a certain range and using, as a suitable immersion liquid, water where an additive of decreasing the surface tension or increasing the surface activity is added. However, the additive is not disclosed or the resist suitable for immersion exposure techniques is not discussed.
Recent progress of immersion exposure techniques is reported, for example, in Proceedings of Society of Photo-Optical Instrumentation Engineers (Proc. SPIE), Vol. 4688, page 11 (2002) and J. Vac. Sci. Technol. B, 17 (1999). In the case of using an ArF excimer laser as the light source, in view of safety on handling as well as transmittance and refractive index at 193 nm, pure water (refractive index at 193 nm: 1.44) is considered to be a most promising immersion liquid. In the case of using an F2 excimer laser as the light source, a fluorine-containing solution is being studied in the light of balance between transmittance and refractive index at 157 nm, but those satisfied in view of environmental safety or refractive index have been not yet found out. Considering the degree of immersion effect and the maturity of resist, the immersion exposure technique is expected to be most soon mounted on an ArF exposure machine.
Since the discovery of resist for KrF excimer laser (248 nm), an image forming method called chemical amplification is used as the image forming method for the resist so as to compensate the reduction of sensitivity due to light absorption. According to this image forming method, for example, in the case of using positive chemical amplification, an acid generator in the exposed area decomposes upon exposure to generate an acid, the acid generated is used as a reaction catalyst in the post-exposure baking (PEB) to convert the alkali-insoluble group (group insoluble in alkali developer) into an alkali-soluble group (group soluble in alkali developer), and the exposed area is removed by an alkali developer.
When a chemical amplification resist is applied to the immersion exposure technique, the acid on the resist surface generated upon exposure moves to the immersion liquid, and the acid concentration on the surface of exposed area is changed. This may be considered to have a close resemblance to the acid deactivation occurring on the surface of exposed area due to basic contamination in a very small amount on the order of several ppb from the environment at the post-exposure time delay (PED) between exposure and PEB, which was a serious problem at the initiation of development of chemical amplification-type positive resist, but the effect of immersion exposure on the resist and also the mechanism thereof are not yet elucidated.
As for the immersion-type scanning exposure machine (scanner), two kinds of scanners have been proposed, that is, one is a full fill-type scanner of immersing the entire wafer in an immersion liquid (also called a bath-type scanner) and another is a local fill-type scanner of introducing an immersion liquid into only a space between the objective lens of exposure machine and the wafer (also called a shower-type scanner). Out of these scanners, the local fill-type scanner is admitted as the standard immersion scanner in view of high throughput and the like.
When exposure is performed by using the local fill-type immersion scanner, the time for which the resist film is in contact with the immersion liquid differs depending on the region in the wafer plane. In other words, a region which is repeatedly immersed in water is generated depending on the position in the wafer plane and this causes a fear that the resist performance is changed depending on the position, that is, the uniformity in the wafer plane is impaired. In practice, it is found that when a chemically amplified resist having no problem in the lithography performance at the process by a dry exposure technique is wet-exposed and after once removing the immersion liquid, subjected to a PEB step by again immersing it in an immersion liquid, the resist sensitivity decreases (has immersion time dependency) as compared with the case where the resist is wet-exposed and as-is subjected to a PEB step.
If a large amount of generated acid dissolves out from the resist surface into the immersion liquid at the immersion exposure, the objective lens of the exposure machine may be contaminated. Therefore, it is demanded to reduce the elution of generated acid as much as possible.